So, the antipaladin's alternative to the standard paladin's disease immunity is kind of a curious critter. It allows the antipaladin to contract diseases just like anyone else would, but he never suffers any damage or penalties from them. Here's the exact text, straight from the PRD for reference:

PRD wrote:

Plague Bringer (Ex): At 3rd level, the powers of darkness make an antipaladin a beacon of corruption and disease. An antipaladin does not take any damage or take any penalty from diseases. He can still contract diseases and spread them to others, but he is otherwise immune to their effects.

Now, I have several questions about this ability. First of all, does an antipaladin continue making Fortitude saves to "fight" the disease just as anyone else would? If this is the case, then the antipaladin will inevitably cure himself of any affliction he happens to pick up (particularly with their formidable Fortitude save) in relatively short order. Can the antipaladin voluntarily choose to accept the disease and waive his chances to fight the infection? What if he is already immune to diseases (by being undead, for example)? Can he still contract them then?

Secondly, once the antipaladin acquires such a disease (or several), how does he spread them? The "diseased" cruelty only functions as the contagion spell, which defines a set list of diseases that can be conveyed. If an antipaladin goes out of his way to contract a particularly virulent or exotic disease for the purpose of spreading it... how can he do that? Does he need to bite people? Cough on people a lot? Are his weapon attacks sufficient to inflict the disease(s) he carries? What about diseases that aren't conveyed through injury? Can he grapple someone and give them leprosy?

Finally, what about diseases that aren't just diseases (like mummy rot)? The conventional paladin gains immunity to even these exotic afflictions, so I am inclined to believe that the antipaladin is similarly immune to their effect but then, can they still be spread? An antipaladin rampantly spreading mummy rot through a town would obliterate the population pretty rapidly and with little hope of stopping the infection once it started.

This ability has a lot of potential to be really flavorful, but it seems like the execution is incomplete. I'd love to hear what others think about how it should be arbitrated.

The First Edition Anti-Paladin, which was designed as an NPC class, had this ability. Very handy. The way we played it out was when the Anti-Paladin wanted to spread the disease all he had to do was touch the victim or victims. The favorite disease of the AP I played was influenza. Wiped out thousands over the course of our campaign (1985 and a DM who was completely over the top all the time).

I don't really see Mummy Rot being all that attractive to have and pass on, usually the things that you fight will be dead in the next 30 seconds anyways, so unless it has a passive buff of some sort, there's no real benefit to having it.

The "Typhoid Mary"-deal is probably something for NPCs anyway; PCs only really benefit from penalties you can inflict during a fight.

A high-Charisma antipaladin could be an effective disease-based assassin, walking into a ballroom or gala, looking every part the dashing hero... and then shake hands with the local king or kiss the hand of an important ambassador, only to have them die several days later from a crippling illness and none would be the wiser.